The rapidly evolving fields of cancer immunotherapy have recently led to the FDA approval of several new immunotherapies, with many more therapies presently in clinical trials for a variety of cancers. Furthermore, cellular, small molecule, antibody-based immunotherapies, and combinations thereof, are being rigorously tested preclinically for clinical translation. The dynamic tumor microenvironment and tumor heterogeneity have become important topics in both preclinical and clinical studies (Hanahan D, Weinberg R A. Cell 2011; 144:646-74; M antovani A, Allavena P, Sica A, Balkwill F. Nature 2008; 454:436-44; Schreiber R D, Old L J, Smyth M J. Science 2011; 331:1565-70.), but the ability to monitor changes in the immune status of primary lesions and metastatic cancers is limited. Current methods to monitor lymphocytes from whole blood or biopsies from heterogeneous tumors do not reflect the dynamic and spatial information likely required to monitor immune responses to therapeutic intervention, many of which elicit whole body changes in immune cell numbers and localization. Therefore, molecular imaging methods that can noninvasively monitor both systemic and intratumoral alterations in immune cell numbers or localization during experimental therapies have the ability to increase the understanding of the dynamics of immunotherapeutic mechanism with the potential to provide translatable methods for predicting and/or assessing clinical immunotherapeutic responses.
Analysis of tumor-infiltrating lymphocytes (TIL) has demonstrated the importance of tumor immune microenvironment and that the presence of cytotoxic CD8+ T cells can predict overall survival in breast, lung, ovarian, melanoma, and colorectal cancers (reviewed in refs. Pages F, et al. Oncogene 2010; 29:1093-102. and Gooden M J, et al. Br J Cancer 2011; 105:93-103.). With the recent clinical successes of immunotherapies that alter the tumor immune microenvironment, including adoptive cell transfer (ACT) of T-cell receptor (TCR)- or chimeric antigen receptor-transduced cytotoxic T cells (Johnson L A, et al. Blood 2009; 114:535-46; Rosenberg S A. Sci Transl Med 2012; 4:127ps8.), agonistic antibodies targeting CD137 (4-1BB) and CD40 (Melero I, et al. Clin Cancer Res 2013; 19:997-1008; Melero I, et al. Nat Rev Cancer 2007; 7:95-106; Vinay D S, and Kwon B S. Mol Cancer Ther 2012; 11:1062-70.), and antibody blockade of the checkpoint inhibitors CTLA-4, PD-1, and PD-L1 (Callahan M K, and Wolchok J D. J Leukoc Biol 2013; 94:41-53; Shin D S, and Ribas A. Curr Opin Immunol 2015; 33C:23-35; Topalian S L, et al. Cancer Cell 2015; 27:450-61.), the ability to noninvasively monitor the tumor immune response to therapy has become of upmost importance.